1. Field of the Invention
This invention relates to optical type displacement detecting devices and more particularly to improvements in an optical type displacement detecting device, wherein a positional relationship between two members is detected from a change of a photoelectrically transduced signal produced by a relative displacement between a main scale formed thereon with an optical grating and an index scale formed thereon with an optical grating corresponding to that of the main scale.
2. Description of the Prior Art
To measure a feed value of a tool of a tool machine and the like in the field of tool machines and measuring instruments, as shown in FIG. 15, there has heretofore been popularized such an optical type displacement detecting device that a main scale 14 formed thereon with a first optical grating (hereinafter referred to as the "first grating") 16 is fixed to one of members movable relative to each other, and a slider having an index scale 18 formed thereon with a second optical grating (hereinafter referred to as the "second grating") 20, illuminating means constituted by a light source 10 and a collimator lens 12 for example, and photoelectric transducing means constituted by light receiving elements 22 for example are fixed to the other member, whereby a change in quantity of light produced by a relative movement between the first grating 16 and the second grating 20 is photo-electrically transduced and detection signals thus detected are interpolated and converted into pulses and counted by an attached counter circuit, so that a displacement value can be measured.
In the above-described detecting device, for example, the second grating 20 formed on the index scale 18 is divided into four including phases 0.degree., 90.degree., 180.degree. and 270.degree. as shown in FIG. 15, and a change in quantity of light is differentially amplified by preamplifiers 24A and 24B, whereby detection signals of two phases which can be substantially approximated by A sin .theta. and A cos .theta. corresponding to a displacement in the direction x of the index scale 18.
In the above-described detecting device, along with advanced finishing techniques, necessity has been voiced for dividing the measuring resolution further smaller. However, there is a limit imposed on the number of interpolation of the detection signal in the counter circuit, and further there is a possibility of errors caused by the interpolation, whereby to further make smaller a pitch of the detection signal itself is demanded, so that a grating pitch P of the first grating 16 of the main scale 14 tends to be decreased. Heretofore, the grating pitch P has been set at about 20 micrometer, however, recently, a specification of 10 micrometer or less is on demand.
However, the following disadvantages have been presented along with reduction of the grating pitch P of the first grating 16 of the main scale 14.
More specifically, from the viewpoint of design of mechanism, it is necessary that an absolute value of a gap g between the main scale 14 and the index scale 18 and an allowable variation thereof should be set at some values or more. However, in order to meet the requirements when the grating pitch P is small in value, it is necessary to use the collimator lens 12 having high accuracy and a long focal length, for making illuminating light to be satisfactorily parallel rays. As a consequence, the detecting device tends to be large-sized.
On the other hand, as a detecting device wherein the gap g can be increased without using the collimator lens 12 with high accuracy, British patent application No. 44522/74 has been proposed. However, in this prior art, such a disadvantage is presented that the detecting device is difficult to be applied to the transmission type detecting device in particular, because three optical gratings are needed.
Furthermore, there is a disadvantage of grating pitch of the second grating 20 formed on the index scale 18 and dividing the phase thereof. More specifically, the second grating 20 is divided into four in the example of FIG. 15. However, in study of the accuracy of a deviation .delta. of vertical dividing, when the grating pitch of the first grating 16 is 8 micrometer, if the grating pitch of the second grating 20 is also 8 micrometer, then, in order to obtain the accuracy of 90.degree. plus or minus 10.degree. as a phase difference of the detection signals, it is necessary to use a very small grating pitch and set the deviation .delta. at (2 plus or minus 0.2) micrometer. As a consequence, advanced finishing techniques are needed and the yield of the index scale 18 in the manufacturing is deteriorated, thus resulting in increased cost.
On the other hand, as another detecting device advantageous in rendering the device with high resolving-power, there is proposed a detecting device for making so-called optical division, wherein a pitch t of a detection signal obtained against the pitch P of the first grating formed on the main scale is one obtained by making the pitch P smaller, such for example as P/2.
In the aforesaid British patent application No. 44522/74 for example, there is disclosed a detecting device wherein the three optical gratings are used to produce a detection signal having a 1/2 pitch of the grating pitch P of the main scale. However, the disadvantage is presented that the prior art is difficult to be applied to the transmission type detecting device as aforesaid.
Furthermore, in British patent application No. 2024416A, as shown in FIG. 16, there is proposed a detecting device, wherein the ratio between a light transmitting portion 16A and a light shielding portion 16B of the first grating 16 formed on the main scale 14 is set at 1: (2n-1), whereby the pitch t of a detection signal obtained by the light receiving elements (photodiodes) 22 through the index scale 18 formed thereon with the second grating 20 is 1/n of the pitch P of the first grating 16.
In the detecting device shown in FIG. 16, the ratio between the light transmitting portion 16A of the first grating 16 and the grating pitch P thereof is set at 1:4, a pitch q of the second grating 20 is set at P/2, and a pitch t of a detection signal obtained by amplifying signals from the photodiode 22 by the preamplifier 24 is q, i.e., 1/2 of the pitch P of the first grating.
By use of the detecting device shown in FIG. 16, a detection signal of the pitch t obtained by dividing the pitch P of the first grating 16 can be positively obtained. However, since it is necessary to make shorter the length of the light transmitting portion 16A of the first grating 16 in proportion to the number of division, this is substantially equal to that the grating pitch is further made smaller, and particularly, from the viewpoint of the finishing techniques, it becomes difficult to manufacture the first grating 16 formed on the main scale 14 having the long measuring range originally. Furthermore, since the satisfactory parallel rays are needed, a collimator lens having high accuracy and a long focal length is required, thus resulting in a large-sized detecting device. Further, against the grating pitch P of the main scale, the grating pitch of the index scale is P/2, and, such a disadvantage is presented that, when the pitch P is 10 micrometer or less for example, the yield of the index scale in manufacturing is low, thus resulting in increased cost.
On the other hand, in the aforesaid British patent application No. 2024416A, further, there is described an example wherein, even when the ratio between the lengths of the light transmitting portion 16A and the light shielding portion 16B of the first grating 16 is 1:1, optical division can be performed. However, in this prior art, when an effective wavelength of an optical system is set at .lambda. and the pitch of the first grating 16 is set at P, the grating gap v needs to satisfy a relationship of the following formula (1), thus presenting such a disadvantage that a mechanism section for holding the grating gap v at a predetermined value becomes complicated. EQU v.ltorsim.(P.sup.2 /.lambda.)/2 (1)